This invention is directed to biconical-taper couplers and, in particular, to multimode fiber couplers.
The fused biconical-taper coupler is described in U.S. Pat. No. 4,291,940, which issued on Sept. 29, 1981, in the name of B. S. Kawasaki, K. O. Hill and D. C. Johnson. This coupler is a widely used type of distribution element for multimode-fiber systems. This device offers the attractive combination of low excess loss, arbitrary coupling ratio, and compatibility with the fiber lines. However, for applications in which many couplers are used in combination, such as data-bus networks, this type of coupler has the drawback of being modal sensitive, that is, the coupling ratio and the excess loss vary with the mode-fill distribution of the light in the input fiber. Furthermore, the mode-fill distribution is changed as the light traverses the coupler. In a network in which several such devices are cascaded, the parameters and performance of a given device are functions of its location within the network. In order to ensure adequate received optical power at all terminals, the system designer is forced to overdesign the network for optical power budget or to build up the network piecemeal while selecting couplers.